Zinc Plating for Steel: Capability Spotlight

Corrosion is one of the most powerful destructive forces challenging human invention and industry. NACE International estimates that the global cost of corrosion is around US$2.5 trillion, or 3.4% of the global GDP. It’s no wonder that people have been experimenting with ways to prevent corrosion for thousands of years. In fact, written records of the treatment of ship bottoms from the fifth century B.C.E. have been found! Early methods of corrosion prevention included anti-fouling paints and copper sheathing until the development of electrochemical coatings in the mid-19th century. 

What is Zinc Plating?

Zinc is used as a sacrificial coating over steel, which is meant to corrode first before the environment attacks the steel itself. Zinc plating is an electrochemical galvanizing technique where a thin coating of zinc metal is applied to the surface of a metal object (known as the substrate). 

The Zinc Plating Process

Cleaning & Surface Preparation

If parts aren’t cleaned of debris and contaminants prior to the plating process, then the zinc coating will not adhere consistently to the parts. Paulo uses a four-step process to clean parts prior to zinc plating:

1. Soaking in an alkaline detergent solution
2. Soaking in an alkaline electrocleaning solution
3. Acid treatment to remove any surface oxides
4. Rinsing

Preparing the Plating Solution

Then, we prepare a specially formulated electrolyte solution that is comprised of zinc metal ions and other chemicals, known as the plating bath. The other elements in the plating solution help lend the desired chemical and physical properties in the end parts. 

There are two types of zinc plating used in metal finishing: acid zinc and alkaline zinc. Paulo’s Murfreesboro division uses an acid process with zinc chloride complexes to apply the plating over steel. When compared to alkaline zinc plating, acid zinc has a faster electroplating rate and is more environmentally friendly, with minimal waste treatment requirements.

Introducing the Electrical Current

Electroplating means that an electrical current is used to deposit the metal ions onto the part surface (the substrate), which serves as the cathode. To apply the electroplated coating, a direct current is applied to the plating tank. This current dissolves the zinc metal anode, putting it into the conductive plating solution. This dissolved zinc is subsequently deposited onto the steel parts, which serve as the cathode, completing the electrochemical process.

Post-Treatment Procedures

After plating, parts are rinsed with water to remove any contaminants or leftover solution. 

Passivation

Trivalent passivates can be applied after plating to improve salt spray performance, enhance appearance, or for part identification. At Paulo, we offer clear (or “blue”) and yellow trivalent passivates. We do not offer hexavalent chrome finishes due to their toxicity, environmental risk, and the demands for REACH- and RoHS-compliant parts. We can also apply other seals that improve salt spray performance.

Hydrogen Embrittlement Relief

Fasteners, springs, and other metal parts requiring a Rockwell hardness of 30 or greater typically require hydrogen embrittlement relief, also known in the metal finishing industry as HE relief baking. Because the cleaning and plating process potentially infuses hydrogen into the parts, in certain operating environments, voids may form in the part microstructure that may lead to brittleness, stress cracking, and ultimately, premature failure of the part. 

HE relief baking is done in bake ovens at a carefully controlled temperature. The delay time between the end of the plating process and the start of the relief baking process is important, and kept to a minimum. Some specifications state it must be initiated within one hour of the plating process and takes a minimum of three hours to complete. Paulo can tailor a bake process to meet the HE relief baking requirements stated in your zinc plating specification.

Salt Spray Testing

Paulo’s Murfreesboro division offers in-house salt spray testing, saving you a critical step in the quality control process for your zinc plated parts.

Zinc Plating Applications

Zinc plating is best suited for large batches of small parts. It is ideal for small fasteners such as screws, nuts, bolts, and other small hardware pieces. It has also gained widespread use in the automotive industry on clips and power steering components.

Our zinc plating lines are optimized for high volumes of small parts and use a barrel plating process. Barrels are ideal for smaller parts because the vessel is rotated during plating, which gives the surface of the parts a more consistent coating. 

Zinc Plating Specifications

Our processes produce results that can conform to the four classes of zinc plating listed in ASTM Specification B 633:

• Fe/Zn 5
• Fe/Zn 8
• Fe/Zn 12
• Fe/Zn 15

The numbers indicate the coating thickness in microns (µm).

Our Murfreesboro division is CQI-11 certified for zinc plating, hydrogen embrittlement relief, and process control and testing.

Heat Treat and Plate Your Parts at Paulo

When you trust your parts to Paulo, you can simplify your processing with heat treating and metal finishing processes like zinc plating under one roof. We understand that if you require zinc plating for your parts, consistency and capacity matter. The heat treating furnaces and zinc plating lines at our Murfreesboro division are all integrated with our PICS system to ensure excellent quality control and proper processing. You’ll also streamline your logistics needs and optimize your turnaround times without the need for transportation to a secondary metal finishing facility once your parts are heat treated.

If you’re searching for a partner who can deliver the quality and turnaround times you need for your parts, we can help. Connect with a Paulo expert today to get started!